Like reptiles, amphibians (frogs, toads, salamanders and their cousins) inhabit most parts of the globe, but around one-third of the nearly 6,000 known amphibian species on Earth currently face extinction. Just as tadpoles turn into frogs, all amphibians undergo metamorphosis between the juvenile and adult stages of their life cycles. More than 20 species of this cold-blooded class are represented at Zoo Atlanta, and many are critically endangered.